build: strip CFG_PREFIX from CFG_{DATADIR,MANDIR,INFODIR,LIBDIR}

build: strip CFG_PREFIX from CFG_{DATADIR,MANDIR,INFODIR,LIBDIR}

While these variables are not yet used by the Makefile, to be used
CFG_PREFIX must be stripped from them. The 'make install' rule creates
a tarball and then the install.sh script extracts it relative to the
prefix argument --prefix, which in the case of a Cargo install is
relative to CFG_PREFIX. This is why CFG_PREFIX needs to be stripped out
of CFG_DATADIR, CFG_MANDIR, CFG_INFODIR, and CFG_LIBDIR.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>